1. Field of the Invention
The present invention relates to a rotary head type magnetic tape recording and reproducing apparatus having plural pairs of rotary heads.
2. Description of the Background Art
In magnetic tape recording and reproducing apparatus for digitizing a sound signal, an image signal and so on to record/reproduce these digitized signals on a recording medium such as a magnetic tape, a digital signal, in general, requires a wider frequency band compared to an analog signal which is not digitized. For record in high density a digital signal having the high-frequency band, such a method is provided the digital signal is recorded by employing a magnetic tape recording and reproducing apparatus using rotary heads to be employed in home video tape recorders and the like.
This rotary head type magnetic tape recording and reproducing apparatus includes an analog/digital converter (hereinafter referred to as A/D converter) for converting a supplied analog signal to a digital signal, a memory having a buffer function for temporarily storing the converted digital signal, a data format generating circuit for forming a predetermined data format based on the digital signal stored in the memory, and a controller for controlling these elements. An output of the data format generating circuit is modulated to be supplied to the rotary heads and then recorded on the magnetic tape lapped around the rotary heads.
A clock generating circuit for generating a plurality of clock signals having different frequencies is provided in the controller. The A/D converter, memory, data format generating circuit, a motor for driving the rotary heads and so forth each have of their operations controlled in response to each of the clock signals generated from the clock generating circuit.
FIG. 7 is a block diagram showing an electrical configuration of a typical conventional clock generating circuit. A clock generating circuit 1 includes a counter circuit 2 to be implemented by, for example, an n-bit binary counter, and two decoders 3 and 4. Counter circuit 2 counts the number of pulses of a reference clock signal to be supplied, so as to apply count outputs of a plurality of bits each having different periods to two decoders 3 and 4.
A sampling clock signal B is outputted from decoder 3 to the A/D converter. Decoder 4 outputs a slot clock signal C to the data format generating circuit and a read clock signal D to the memory. A most significant bit signal from counter circuit 2, i.e., a signal obtained by frequency-dividing the reference clock signal by a frequency dividing ratio of 2.sup.n is applied as a synchronizing clock signal A of the rotary heads to the motor.
FIG. 8A is a diagram schematically showing a rotary head apparatus 5 on which plural pairs of heads are mounted. A rotary drum 8 having a diameter R0 is mounted with two magnetic heads 6a and 6b for recording and two magnetic heads 7a and 7b for reproduction. A spacing of 90.degree. (the angle of mounting) is provided between any adjacent recording magnetic heads 6a and 6b and reproducing magnetic heads 7a and 7b. One reproducing magnetic head 7a is used as a monitor head for reproducing data recorded on a magnetic tape 9 by one recording magnetic head 6a immediately after the data is recorded and then checking whether or not the data is correctly recorded on the tape. Similarly, the other reproducing magnetic head 7b is used as a monitor head corresponding to the other recording magnetic head 6b.
Magnetic tape 9 is lapped around rotary drum 8 at a lap angle .alpha.1 (90.degree. in FIG. 8A). Magnetic tape 9 moves at a speed Vt in the direction of an arrow F, while rotary drum 8 rotates at a rotating speed Vd in the direction of an arrow G. Since tape moving speed Vt is sufficiently low compared to drum rotating speed Vd, a tracing speed at which each of magnetic heads 6a, 6b, 7a and 7b traces on magnetic tape 9 is approximately equal to drum rotating speed Vt.
FIG. 8B is a diagram showing to timing of each magnetic heads 6a, 6b, 7a and 7b traces on magnetic tape 9. In a period T1 in which rotary drum 8 makes one revolution, one of magnetic heads 6a, 6b, 7a and 7b starts tracing corresponding track each time rotary drum 8 rotates 90.degree.. Since the mount angle of each magnetic head and the lap angle .alpha.1 of magnetic tape 9 are both 90.degree. in FIG. 8A, a time that a preceding magnetic head ends tracing is coincident with a time that the succeeding magnetic head starts tracing. In FIG. 8B, a designation W1 denotes a signal recorded by recording magnetic head 6a; W2 denotes a signal recorded by recording magnetic head 6b; R1 denotes a signal reproduced by reproducing magnetic head 7a; and R2 denotes a signal reproduced by reproducing magnetic head 7b.
It is now assumed that the diameter R0 of rotary drum 8 is decreased. FIG. 9A shows a rotary drum 8a having a diameter Ra two-thirds that of rotary drum 8 of FIG. 8A. When the rotating speed of rotary drum 8a is kept equal to that of rotary drum 8, the circumferential speed of rotary drum 8a decreases due to the reduction in diameter, and hence the tracing speed of magnetic heads 6a, 6b, 7a and 7b with respect to magnetic tape 9 decreases compared to the magnetic heads of rotary head apparatus 5 shown in FIG. 8A. That is, a distance that magnetic heads 6a, 6b, 7a and 7b trace on magnetic tape 9 is decreased. Thus, if the lap angle .alpha.1 of magnetic tape 9 is increased to a lap angle .alpha.2 as shown in FIG. 9A wherein the angle corresponds to a decrease in tracing speed, the distance for which each of magnetic heads 6a, 6b, 7a and 7b traces the magnetic tape 9 during one revolution of the rotary drum 8a can be kept equal to that in rotary drum 8. Accordingly, the lap angle .alpha.2 of magnetic tape 9 with respect to rotary drum 8a is selected to be 135.degree.. However, even if the lap angle of the magnetic tape is increased, the tracing speed of each magnetic head is reduced as described above, so that a rotary head apparatus 5a of FIG. 9A cannot record information on magnetic tape 9 at the same density as rotary drum 8.
Thus, if the frequencies of slot clock signal C and read clock signal D of the data format generating circuit are lowered by a proportion corresponding to the decrease in diameter R0 without any change in synchronizing clock signal A of rotary head 8a and in sampling clock signal B of the A/D converter, rotary head apparatus 5a of FIG. 9A can perform recording/reproduction in the same recording format as in rotary head apparatus 5 of FIG. 8A. This makes it possible to reduce a reading speed at which data is read out of the memory without changing the rotating speed of rotary drum 8a and consequently generate a desired recording format in response to the changed clock slot signal C.
FIG. 9B shows the timing of each magnetic heads 6a, 6b, 7a and 7b tracing the magnetic tape 9 during one revolution of rotary drum 8a. As shown in FIG. 9A, when the mount angle between any adjacent magnetic heads is 90.degree., while the lap angle .alpha.2 of magnetic tape 9 is selected to be 135.degree., a preceding magnetic head tracing the magnetic tape 9 ends tracing after the succeeding magnetic head starts tracing. Accordingly, a recording period in which the signal is recorded and a reproducing period in which the signal is reproduced overlap with each other. FIG. 9B shows an overlapping period Z1 of recorded signal W2 and reproduced signal R1 and an overlapping period Z2 of recorded signal W1 and reproduced signal R2. In these overlapping periods Z1 and Z2, an S/N ratio (signal to noise ratio) of the reproduced signal is degraded due to cross talk in the recorded signal having a great signal strength.
In order to prevent cross talk, as shown in FIG. 10A, the mount angle between recording magnetic heads 6a and 6b disposed on rotary drum 8a and that between reproducing magnetic heads 7a and 7b are decreased. That is, since the lap angle .alpha.2 of magnetic tape 9 is selected to be 135.degree. in FIG. 10A, 45.degree. setting of the mount angle between any magnetic heads suppresses the generation of the overlapping periods of the recorded signal and the reproduced signal, as shown in FIG. 10B, so as to prevent cross talk.
Meanwhile, the timing of each magnetic head tracing recorded signals W1 and W2 or reproduced signals R1 and R2, as shown in FIG. 10B, is controlled by a clock signal obtained based on a reference clock signal CK in each clock generating circuit 1 shown in FIG. 7 provided for each magnetic head.
In the conventional rotary head type magnetic tape recording and reproducing apparatus, if a rotary drum having a specific head position is designed, a clock generating circuit corresponding thereto must be designed separately. It is thus impossible to make an arbitrary alteration of head position of the rotary drum as far as the configuration of the clock generating circuit is not altered. In other words, if the clock generating circuit is designed with respect to a specific head position, it is impossible to alter timing for generating a clock signal in accordance with alteration of the head position of the rotary drum. Therefore, the conventional rotary head type magnetic tape recording and reproducing apparatus is unable to perform recording/reproduction using the same data format in accordance with the alteration of the head position of the rotary drum.